1. Field of the Invention
This invention relates to static high power inverters in which waveforms generated in a plurality of phase displaced bridge inverter circuits are phase shifted by transformers to produce a composite waveform having a pulse count equal to the sum of the pulses generated by the separate bridge circuits and with reduced harmonics. More particularly, the invention is directed to such static high power inverters in which the transformer requirements are reduced, both in the number and complexity, while maintaining the quality of the quasi-harmonic neutralized composite waveform.
2. Background Information
Static inverters employing semiconductor switching devices are now commonly used for many applications including industrial drive, power conditioning, dc-link frequency conversion and the generation of controllable leading or lagging reactive current.
For low power applications, fast switching transistors can be used with high frequency switching or pulse width modulation techniques to realize high quality sinusoidal output waveforms. For larger power ratings where high frequency switching becomes less efficient, programmed waveform techniques are employed to produce a good quality waveform with a reduced number of switchings in each fundamental period.
For very high power applications where many large semiconductor switches are required and where efficiency is important, the switches are operated at fundamental frequency and harmonic neutralizing techniques using special transformer configurations are used. A high quality multi-step output waveform is derived by combining outputs from a number of inverter stages each operating at fundamental frequency. Each step of the output waveform is evenly spaced and has an amplitude proportional to the sine of its angular position. The number of steps is referred to as the pulse number.
In this type of multi-step or multi-pulse inverter, each switching device operates with identical voltage and current waveforms and contributes equally to the output. Because all switches turn on and off at the same levels of current, all devices operate with similar delays and the effects of differences in current dependent switching delays upon the output waveform are minimized.
The harmonic spectrum of the synthesized output contains terms having harmonic orders: EQU H=np.+-.1
and amplitude: EQU Ah=1/(np.times.1)
where
p is the pulse number and n is any integer.
If a high quality waveform is desired, a high pulse number is clearly an advantage.
The simplest threephase harmonic neutralized inverter is the six-pulse bridge inverter circuit. This consists of three inverter poles connected across a dc voltage source. Each pole has two switching devices connected in series, the junction of the switches being the ac output terminal.
The three inverter poles each operate at fundamental frequency and produce three square wave outputs with respect to the midpoint of the dc voltage. The three outputs are symmetrically displaced by 120 degrees so that a pole transition occurs every 60 degrees, or in other words, there are six state changes in a cycle of fundamental frequency. The outputs voltages produced between the three ac terminals are true six-pulse waveforms. Requiring no transformer, the six-pulse bridge inverter forms the basic building block generally used to make up higher pulse number inverters.
To produce true harmonic neutralized outputs having pulse number of N.times.6, the outputs of N six-pulse bridges are combined as follows:
1. The bridges are operated from a common dc source with their outputs incrementally phase displaced by an angle which corresponds to one segment of the desired multi-segment or multi-pulse output. That is a displacement angle of 360/6N degrees.
2. The phase displaced fundamental outputs of individual six-pulse bridges are shifted into phase with each other by individual transformers having appropriate winding configurations and the same voltage ratios.
3. The transformed outputs of each bridge which now have the same fundamental amplitude and phase are combined, either by series connection of the secondaries, by parallel connection through appropriate interphase transformers, or by some combination of series and parallel connection.
To derive the necessary phase shifts, transformers having differently configured windings are required to interconnect the six-pulse building blocks. For a given pulse number, many different transformer configurations could be derived which meet the requirements for true harmonic neutralization. Some of these transformer configurations are non-standard requiring specially fabricated transformers which add significantly to the cost of the inverter. While the improvement in waveform quality obtainable with higher pulse numbers is significant, the increased complexity and cost of the special transformers cannot be justified for most applications.
In practice, the simple winding configurations are preferred and 12-pulse configurations employing wye/wye and delta/wye windings are most common.
Several techniques have been used to combine the outputs of six-pulse bridge inverters to produce a greatly improved "quasi-harmonic neutralized" output with less complicated transformer configurations.
Two basic approaches are:
(a) Employ two or more pairs of wye/wye and wye/delta transformers each fed by a pair of six-pulse bridges to form 12-pulse outputs, each 12-pulse output being generated with an appropriate displacement and the outputs being combined by series connection of the transformer secondaries. The resultant 12-pulse outputs having a small difference in phase combine to produce an output having evenly spaced steps with slight amplitude differences from a true harmonic neutralized waveform. This technique allows readily available standard transformer configurations to be employed for systems having pulse numbers of 24, 36, 48 and so on.
(b) Employ half the number of transformers required for a true harmonic neutralized system, having phase shifts corresponding to a system having half the desired pulse number, and having open wye or zig-zag primaries. Feed the opposite ends of each set of primary windings from two nearly oppositely phased six pulse bridges so that the wave form applied to each transformer primary windings is a quasi-square wave whose amplitude remains at zero for an interval corresponding to one step of the final multi-pulse output. The fundamental phase of the output of each pair of six-pulse bridges is selected to match the fundamental phase of the transformer primary windings when the secondaries are directly in-phase.
An example of an inverter employing method (a) comprises four six-pulse bridge inverters divided into pairs which are connected respectively to wye/wye and delta/wye transformers to produce 12-pulse outputs. The two 12-pulse outputs are displaced so that the first leads and the second lags the final output by 7.5 degrees. The outputs are summed in series to produce a 24-pulse output.
An example of method (b) comprises two identical transformers having wye primaries and 15 degree shift zig-zag secondaries fed by two pairs of six-pulse inverters. The first pair of inverters are operated at angles of 22.5 and (7.5 +180) degrees and feed the opposite ends of the open wye primaries of the first transformer. The second pair of inverters operated at angles of -7.5 and (-22.5 +180) degrees feed the opposite ends of the open wye primaries of the second transformer. The fundamental voltages produced at the secondaries are both at zero degrees and sum directly to form a 24-pulse output.
The first technique (a) does not reduce the total number of transformers required.
The second technique (b) is restricted to phase-shifting transformers which do not have delta windings or other connections which form a short circuit to zero sequence voltages which are generated with this technique.
Resolution of the voltages in a threephase electrical system into positive, negative and zero sequence voltages is a known technique of analyzing such systems. The zero sequence voltages are components of each of the threephase voltages which are in phase and equal magnitude. In a true harmonic neutralized inverter, there are no zero sequence voltages. However, in the quasi-harmonic neutralized inverters with open wye primary windings fed at opposite ends by poles of two different six-pulse bridge inverters, zero sequence voltages are present. If the secondary windings of the phase shifting transformers were connected so as to provide a low impedance path for the zero sequence voltages, large zero sequence currents would flow. Unfortunately, while a wye secondary winding in one phase shifting transformer and a delta secondary winding in a second, provide a simple means of achieving the required 30 degree phase shift for a 24-pulse quasi-harmonic neutralized inverter using standard transformers, such an arrangement would provide a short circuit for zero sequence currents generated in an inverter with open wye primaries fed at opposite ends by separate six-pulse bridge inverters circuits. In addition, wye connected secondaries with a neutral line would provide a short circuit for zero sequence currents in this type of quasi-harmonic neutralized inverter.
There is a need therefore for an improved quasi harmonic neutralized static power inverter which requires a reduced number of preferably standard types of phase shifting transformers to generate quality output waveforms.
There is a further need for such an quasi-harmonic neutralized inverter which can be used with a threephase, four line system.
There is an additional need for such a quasi-harmonic neutralized inverter which can be implemented with minimum number of standard single phase transformers.